Conceptual Physics

of the observations they make. Both sets of observations are equally valid, given the

reference frames of the observers, but they do differ. They provide a starting point in

special relativity: Reference frames play an essential role in the observations made by

observers.

An observer

Person who records where, when event

occurs

Uses reference frame to make

measurements

Sara: Foot met ball at 2.8 meter marker

at 5:00 P.M. PST

Observer in different reference

frame

Observers in different reference frames

make different observations

Professor: Foot met ball at position 0.7

meters at 8:00 P.M. EST

35.3 - Light can travel through a vacuum

As the 20th century dawned, the topic of how light travels generated much debate in the

physics community. The evidence brought to bear on this debate helped Einstein to

confirm one of his central insights: The speed of light in a vacuum is constant. The

speed of light does not depend on the motion of its source or of the observer. This

conclusion is one of Einstein’s two postulates; the experiments that enabled him to

deduce this are an interesting story in the history of science.

The primary experiment that propelled Einstein toward his conclusion was not intended

to lead to conclusions about the speed of light. Rather, it was an experiment about the

existence and nature of the medium through which light travels. Earlier observations

had convinced scientists that light acted as a wave. Experiments had shown that a

beam of light spreads out (diffracts) in the same manner as a wave of water, and that

light produces interference patterns that are consistent with the patterns caused by

waves as well.

Since physicists knew light acted as a wave, they went in search of the medium in

which it traveled. They reasoned that waves always move through some form of

medium í water, the wire of a Slinky®, the strings of a violin. However, the medium for

light was mysterious. Scientists were puzzled by the fact that light travels through the

near vacuum of space, where there apparently is no medium. Physicists assumed there must be a medium, and called this elusive medium the

ether. They assumed the ether permeated the universe, including the Earth’s atmosphere.

Some physicists were skeptical about the existence of the ether. Nonetheless, they looked for ways to measure its attributes. Two clever

American physicists, Albert Michelson and Edward Morley, conducted an experiment that played a key role in proving that ether didnot exist.

Michelson and Morley reasoned: If light travels in a medium, it should move faster when traveling in the same direction as that medium and

slower when traveling against it. In other words, light should act like a swimmer in the ocean: An observer on the ground would see the

swimmer moving faster when swimming with the current and slower against it.

Light travels through vacuum

If light traveled through ether

·It would travel faster when moving with

the ether

(^640) Copyright 2007 Kinetic Books Co. Chapter 35